Zeolites are crystalline aluminosilicates that are microporous and which are formed from corner-sharing SiO4/2 and AlO4/2 tetrahedra. Many zeolites, both naturally occurring and synthetically made, are used in various industrial processes, such as catalysis and/or separations. Synthetic zeolites are typically prepared via hydrothermal synthesis in which suitable sources of Si, Al are combined with structure directing agents (SDAs) such as alkali metal cations, alkaline earth metal cations, organoammonium cations, or amines. The SDAs reside in the zeolite pores and are primarily responsible for the particular structure that is ultimately formed. These species can balance the framework charge associated with the AlO4/2 tetrahedra and can also serve as space fillers.
Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces as well as on internal surfaces within the pore.
Zeolites can also be used for separations processes, in which a certain molecule can be readily adsorbed within the zeolite pore structure while another molecule may be rejected or excluded. The use of zeolites in separations takes advantage of the molecular sieve effect, in which maximum size of a molecule that can enter the zeolite pore structure is dictated by the size and shape of the pore openings or apertures. The size of the pore openings is typically defined by number of T-atoms (i.e., Si or Al atoms) that form a closed loop. For example, an 8-ring material contains closed loops of eight corner-sharing SiO4/2 and/or AlO4/2 tetrahedra. 8-ring materials, in particular, have been of considerable interest for small molecule separations, including carbon dioxide capture. When zeolites are used in separation applications, it is advantageous to fabricate them into a membrane. In order to fabricate the highest-quality membranes, it is preferred that the zeolite crystals are as small as possible.